1. Technical Field
The present invention relates to printing apparatuses such as a facsimile apparatus, a copying machine, and a printer for OA equipment, and, more particularly to a printing apparatus, a printing program, a printing method, an image processing apparatus, an image processing program, an image processing method, and a recording medium having the program recorded therein that are suitable for performing print processing of a so-called ink jet system adapted to eject particles of liquid inks of plural colors onto a printing sheet (a recording material) to draw predetermined characters and images thereon.
2. Related Art
A printing apparatus, in particular, a printer adopting the ink jet system (hereinafter referred to as “ink jet printer”) will be hereinafter explained.
In general, it is possible to easily obtain inexpensive and high quality color prints with an ink jet printer. Thus, ink jet printers are widely used not only in offices but also at homes of general users in accordance with the spread of personal computers and digital cameras.
In such an ink jet printer, in general, a movable body called a carriage or the like integrally provided with an ink cartridge and a print head ejects (jets) particles of liquid inks in a dot shape from nozzles of the print head while reciprocating on a print medium (a sheet) in a direction orthogonal to a sheet feeding direction of the print medium. In this way, the ink jet printer draws predetermined characters and images on the print medium to create a desired print. The carriage includes ink cartridges of four colors (yellow, magenta, cyan, and black) and print heads for the respective colors. Thus, the ink jet printer can easily perform not only monochrome printing but also full-color printing in which the respective colors are combined (moreover, full-color printing of six, seven, or eight colors including light cyan, light magenta, and the like in addition to the four colors is also put to practical use).
In the ink jet printer of the type adapted to execute printing while causing the print head to reciprocate on the carriage in the direction orthogonal to the sheet feeding direction in this way, it is necessary to cause the print head to reciprocate several tens of times to one hundred times or more in order to clearly print characters and images on one entire page. Thus, the ink jet printer has a disadvantage in that a substantially long printing time is required compared with printing apparatuses of other systems, for example, a laser printer and the like that use an electrophotographic technology used in a copy machine and the like.
On the other hand, in an ink jet printer of a type in which a long print head having a dimension (width) the same as (or longer than) a width of a printing sheet is arranged and a carriage is not used, it is unnecessary to move the print head in a width direction of the printing sheet. Thus, printing in a so-called one scanning (one path) is possible. This makes it possible to perform high speed printing in the same manner as the laser printer. Further, since the carriage mounted with the print head and a driving system and the like for moving the carriage are unnecessary, this ink jet printer also has an advantage in that it is possible to reduce the size and weight of a printer housing and noise reduction is significantly improved. The ink jet printer of the former type is generally called a “multi-path printer” and the ink jet printer of the latter system is generally called a “line head printer”.
In the print head that is indispensable to such ink jet printers, fine nozzles having a diameter of about 10 μm to 70 μm are disposed in one row at fixed intervals or plural rows in a printing direction. Thus, for example, ink ejecting directions of a part of the nozzles may be slanted because of a manufacturing error or the nozzles may be arranged in positions deviating from ideal positions. As a result, arriving positions of dots formed on a print medium by the nozzles may deviate from ideal positions. In other words, a so-called “flight deviation phenomenon” may occur. Further, because of a dispersion characteristic of the nozzles, in nozzles having a large dispersion, an amount of ink may be extremely large or small compared with an ideal amount.
As a result, a printing failure called a “banding (streak) phenomenon” may occur in a portion of an image printed using the defective nozzles to significantly deteriorate print quality. When the “flight deviation phenomenon” occurs, distances among dots ejected by adjacent nozzles become non-uniform. A “white streak” (when a printing sheet is white) is caused in a portion where the distances among the adjacent dots are longer than a normal distance. A “thick streak” or “dark streak” is caused in a portion where the distances among the adjacent dots are shorter than the normal distance. When a value of an amount of ink deviates from an ideal value, the thick streak is caused in a portion of the nozzles where the amount of ink is large and the white streak is caused in a portion where the ink amount is small.
In particular, such a banding phenomenon tends to occur markedly in the “line head printer” in which the print head or the print medium is fixed (one path printing) rather than the “multi-path printer” (a serial printer) described above (in the multi-path printer, there is a technique for making banding less conspicuous by making use of the fact that the print head can be caused to reciprocate many times).
Therefore, in order to prevent a kind of printing failure due to such a “banding phenomenon”, hardware research and development such as improvement of a technique for manufacturing print heads and design improvement are earnestly carried out. However, because of manufacturing costs, technical issues, and the like, it is difficult to provide a print head that does not cause any “banding phenomenon”.
Under the present situation, in addition to the improvement in hardware, a technique for reducing such a “banding phenomenon” using a so-called software method such as printing control described below can be adopted.
For example, in JP-A-2001-177722, in quantizing a multi-gradation image, after error diffusion processing is carried out, dither processing is performed to cause dots to concentrate in low and medium concentration parts of the multi-gradation image and cause dots to disperse around the concentrating dots in a high concentration part. Consequently, an image without a white void in, in particular, the high concentration part is formed.
However, in the related art described above, although it is possible to reduce the banding phenomenon in the high concentration part, in general, it is difficult to reduce the banding phenomenon in the medium concentration part and the low concentration part where the banding phenomenon is conspicuous. Since the concentration of dots is conspicuous in the medium concentration part and the low concentration part, it is likely that granularity is conspicuous to deteriorate image quality.
In the dither processing, in general, a concentration value of an input image is set on and off with a threshold value of a dither matrix as a boundary. Thus, an input concentration value and an output concentration value may not coincide with each other. Therefore, the dither processing has a characteristic that, in particular, gradation reproducibility is low.